There are many methods of preventing or slowing down the corrosion of silicon carbide products working at elevated or very high temperatures in an oxygen containing atmosphere. The heretofore considered corrosion proceeds according to the formula EQU SiC+2O.sub.2 .fwdarw.SiO.sub.2 +CO.sub.2
In cases where for technological reasons products made of silicon carbide are to be used methods of preventing or slowing down the corrosion by diverse coatings or shields are applied. The Morganite International Limited Company recommends in prospects heating elements made of silicon carbide with coatings of D or L type glazes. A coating with a glaze of the entire surface of diverse ceramic articles working at high temperatures as parts of gas turbines and similar is described in the U.S. Pat. No. 4,159,357. The U.S. Pat. No. 2,003,592 applies as coat a layer of fine silicon carbide crystals bound with a binding agent. The U.S. Pat. No. 4,226,914 discloses a tightly adherent coating on metallic or ceramic substrates, made by plasma spraying of a mixture of finely divided silicon carbide and silicon to be used for rocket nozzles and in other areas where extremely high temperatures are present. According to GFR Pat. No. 2,533,895 the method consists in impregnating the products with a refractory material, preferably containing molybdenium and silicon bisilicide. Porosity of the product is decreased from 30 percent to approx. 5 percent, ensuring a certain degree of resistance to corrosion. The GDR Pat. No. 136,611 includes a two-step impregnation of silicon carbide heating elements. The U.S. Pat. No. 2,943,008 concerns batts supporting ceramic ware in kilns during burning, made among others from silicon carbide, wherein the upper or both the upper and bottom surfaces are covered with a protective ceramic layer to prevent rapid oxidation. All these methods of protecting silicon carbide products aim to prevent or slow down the process of corrosion by oxygen of porous ceramic materials, as silicon carbide among others, working in elevated temperatures. That kind of corrosion occurs in the whole porous body of unprotected products made from silicon carbide. The protective coats and shields substantially limit the oxygen penetration into the pores inside the product.
As we have experienced and now we disclose herewith that under conditions of limited penetration of oxygen into the pores of a silicon carbide products, caused by the above mentioned and not perfectly gestight coatings and shields, another kind of corrosion of silicon carbide products takes place inside the body of the product on places distant from the surface exposed to the oxygen containing atmosphere. This different kind of corrosion of silicon carbide products appears at much lower temperatures than the previous one, it leads to a very rapid destruction of the product and is caused by carbon monoxide formed in the pores under conditions of a very small oxygen concentration. Thus the application of protective coatings and shields, which are not perfectly gastight or have at least a weak point or get rents with time, is extremely detrimental to silicon carbide products.
Below is presented the process of corrosion of silicon carbide products in an oxidizing atmosphere and under conditions of a limited penetration of oxygen into the pores of the product, that is in the case of such an installation of the product that a portion of the total outer surface of the product has been isolated from oxygen to air, as e.g. in a crucible for melting metals the outer face of which is heated, in a thermocouple shield immersed in metal or in a zinc rectification column of a New Jersey system or in the side linings of alumina reduction cells and many other applications.
Under conditions of a limited penetration of oxygen to the pores in the product carbon monoxide is produced as a result of the action of the small concentration of oxygen on trace amounts of carbon always present in silicon carbide products, thus leading to the formation of a strongly reducing atmosphere.
The carbon monoxide reacts with silicon carbide according to the following formulae ##EQU1##
As a result of above reactions silicon dioxide and silicon are produced from a portion of the silicon monoxide and the remaining silicon monoxide is transformed into a solid phase of a brown colour.
The liberated carbon reacts in turn with existing oxygen of low concentration producing further carbon monoxide, which in turn forms silicon monoxide and liberates carbon. Mixture of the listed compounds remains partially in the pores of the product, whereas the remaining portion is deposited on the outer faces of the product. The process leads to a very rapid destruction of the product by rents and cracks.
The rate of corrosion depends first of all on the manner of installation of the product and on its average temperature of work.
According to this pattern the corrosion of silicon carbide products starts already at a temperature of about 550.degree. C.